Carbon-bonded refractory casting mold and process for fabrication thereof



United States Patent 3,153,825 CARBDN-BGNEBED REFRACTGRY CASTING Mil/LBAND PKG-JESS FQR FABREQATIQN TEEEREDF Bryant W. Crochet, Reading, Mass,assignor to the United States of America as represented by the Secretaryof the Army No Drawing. Filed Jan. 8, 1962, Ser. No. ltiSfiM-S 4 Claims.(3. 22193) (Granted under Title 35 US. Qode (1952}, sec. 26%) Theinvention described herein may be manufactured and used by or for theGovernment for governmental purposes without the payment to me of anyroyalty there- This invention relates to the forming of the molds andcores employed in foundry casting operations and is directed moreparticularly to a method of treating resinbonded refractory sands, aswell as metal powders, to provide relatively fine-grained molds ofsuperior strength and surface finish which will not react with themolten metal being cast therein.

Considerable difiiculty has been encountered in the development ofsuitable molds and cores of the type utilized in the precision castingof refractory metals such as titanium and uranium which possess anunusual amnity for oxygen. While the major source of oxygencontamination can be eliminated through the expedient of melting andpouring the metal to be cast in a vacuum, the successful precisioncasting of chemically active refractory metals is still faced with theproblem of developing an inert mold which can be easily and economicallyfabricated. Extensive research in this area has shown that the minimumcontamination in the metal being cast is achieved through the use ofvarious forms of carbonaceous material. Consequently, current practicein the fabrication of precision casting molds and cores is to machinethem from solid blocks of graphite. However, this procedure is not onlyextremely costly but produces molds of poor permeability and tends topromote the formation of cold shuts or laps during the solidification ofthe metal being cast. While some of these undesirable characteristicscan be avoided through the utilization of graphite which is rammed intothe mold configuration and thereafter sintered, the processing of thistype of material is not only expensive but is subject to the problem ofdimensional sensitivity.

It has been found that the use of relatively thin-walled disposablemolds composed of a refractory sand or powder held together by athermosetting plastic or resin is particularly well suited to theproduction of precision castings. However, these molds, which aregenerally referred to as shell molds, are not presently employed invacuum casting in view of the large volume of gases and other volatileswhich are evolved during the burning to which the resin binder issubjected upon contact with the molten metal being cast. The properescape of these gases requires a mold material of relatively highpermeability which, of course, involves a corresponding reduction in theattainable strength thereof.

Accordingly, it is a pr ncipal object of this invention to provide arelatively simple and inexpensive process of forming the molds and coresutilized in precision casting techniques such that the finished productmay be successfully employed in vacuum casting without imparting anysignificant contamination to the metal being cast therein.

A further object of this invention is to provide a more economicalmethod of forming thin-Walled shell molds capable of imparting a bettersurface finish and dimensional stability to vacuum castings than hasbeen heretofore possible with molds of graphite construction.

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Still another object of the present invention is the provision or" aprocess wherein either a refractory sand or metal powder is bonded by athermosetting resin and treated n a vacuum to eliminate any subsequentrelease of combustion gases during contact with the metal being cast.

A particular object of this invention lies in the provision of a methodfor converting the resin bond of a shell-type refractory sand mold to acarbon bond of considerably greater strength without any significantchange in dimensions.

it has been discovered that the foregoing objects may be achieved bysubjecting a shell mold of zircon bonded with phenol-formaldehyde to atemperature of about 165i)" F. within a vacuum of about 20 microns for aperiod of time sufficient to drive 05 all gases and other volatflesevolved during the conversion of the resin to carbon. Such treatmentresults in an unexpected improvement in the molds and cores which areutilized in the precision casting of such refractory metfls as titaniumand uranium since it pr vides a strong, smooth material of variablepermeability which will not significantly contaminate the metal beingcast thereagainst.

Present-day shell molding techniques generally utilize silica in theform of sand as the refractory material from which the mold isfabricated, However, inasmuch as silica will react with carbon and formsilicon and carbon dioxides at the temperatures and vacua required tocompletely carbonize the resin, the present invention utilizes zircon asthe preferred refractory for the mold material. However, experimentationhas shown that other refractory silicates, oxides, as Well as metalpowders, may be employed as a satisfactory substitute for the zircon.For example, it has been found that copper is one of the metals whichwill not seriously contaminate the type of refractory metal to be castand, consequently, when employed in suitable powder form will produceresults which are at least equivalent to those obtained with zirconsand. The size of the individual grains of the refractory material ormetal powder is, of course, dependent upon the degree of surface finishdesired in the solidified castings. if an extremely fine finish isrequired, the grain size of the zircon should be reduced to provide aflour rather than the sand type in normal usage.

While any thermal or air-setting organic plastic may be employed to bindthe refractory material, it has been found that the phenol-formaldehyderesin generally utilized in shell molding is preferred f r reasons ofeconomy. However, extremely successful results have been achieved With afuran air-setting resin and a catalyst which enables the resin topolymerize at room temperature. The resin may be either wet or dry butin the event it is utilized in the latter form, it is essential that theingredients be thoroughly mixed to provide substantially uniform distribution.

Although the percentages by weight of the resin and zircon will vary inaccordance with the particular characteristics desired in the mold,maximum strength and hardness has been achieved with a mixture of 88%zircon flour and 12% resin. A mixture of 45% zircon hour, 45% zirconsand of AFS No. fineness, and 16% resin has also given excellentresults. If greater mold permeability is desired, this may beaccomplished with a mixture of 93% zircon sand of AFS No. 130 finenessand 7% resin. The important characteristics obtained with these mixturesare listed below in T able 1 and are compared with the correspondingresults achieved with a standard shell mold and an oil core mold.

As shown in this table, the fiber stress or transverse strength of themolds fabricated from the mixture of 88% Zircon sand and 12% resin isvirtually double that provided by the shell molds treated in aconventional manner.

Once the selected mixture has been mulled to the desired extent, theshell molds may be formed to shape and cured in the conventional foundrymanner. However, instead of subjecting the molds to the customary bakingprocedure, they are placed in a vacuum furnace whose controls are set toprovide a vacuum of about 20 microns. While 10 to 30 microns constitutesa preferred range, satisfactory results can be achieved with vacuumsystems ranging as high as 1000 microns. Thereafter, the furnacetemperature is raised to about 1650' F. Experimentation has shown thattemperatures below 1400 F. will not provide the desired completecarbonization of the resin while temperatures above2000 F. will resultin an undesirable chemical reaction between the zircon and the cmbonwhich is formed by the decomposition of the resin. The desired furnacetemperature is maintained until operation of the vacuum pump signalsthat the carbonization of the resin has been completed and that nofurther combustion gases will be evolved. The duration of this heatingstep is, of course, dependent upon the size and section thickness of themold. Thereupon, the furnace temperature is lowered to about 400 F. topermit the molds to cool sufficient y for subsequent removal from thefurnace. At this point, the molds can be prepared in the usual manner toreceive the molten metal to be cast therein.

In view of the absence of atmospheric pressure on the mixture of zirconand resin, the rapid evolution of the hydrocarbons and other volatilesproduced by the burning of the resin is accompanied by an unusualfrothing and foaming action which thoroughly distributes the carbonresidue among the individual grains of the zircon sand. Although it isnot definitely known whether the carbon and zircon undergo a chemicalreaction or a physical diffusion, it is apparent that the intimatecontact between the carbon and zircon results in actual bonding at theinterface surfaces thereof. The temporary binding action of the resin isthus converted to an interconnected skeletonized carbon structure whichsurrounds and holds the zircon grains so as to form an unusually strongand rigid mold. inasmuch as the carbonizing temperature is firstattained at the outer surface of the mold and then gradually pervadesthe inner regions thereof, the aforementioned foaming action of theresin binder occurs on a progressive local basis which does not alterthe external dimensions of the mold to any measurable extent.

Chemical analysis of various molds produced in accordance with thisinvention has shown that the resin is almost entirely reduced toamorphous carbon with the retention of only 0.085 percent by weight ofhydrogen. As a result, either titanium or uranium may be cast into thesemolds with no significant surface contamination. Furthermore, unlikeconventional shell molds, the structural rigidity of the mold itselfwill not be seriously weakened by the high temperatures of the moltenmetal being poured therein. Moreover, the absence of gas formationduring the pouring of the casting permits an appreciable reduction inthe permeability required for the mold material thereby enabling the useof a much finer particle size for the zircon which provides a much finersurface finish as well as greater strength and rigidity. It has beenfound that molds prepared in accordance with this invention may also beused for the casting of bronze, copper, aluminum, magnesium, and varioussteel alloys with excellent results.

It can, therefore, be seen that a resin-bonded mold of refractory sandor even a metal powder treated in a vacuumfurnace in the mannerdescribed above will provide sound precision castings in a variety ofmetals over a wide range of casting temperatures.

Although a particular embodiment of the invention has been described indetail herein, it is evident that many variations may be devised Withinthe spirit and scope thereof and the following claims are intended toinclude such variations.

I claim:

1. A process for treating shell molds formed of a mixture of zircon sandand a thermosetting resin polymerized into a temporary binder for thesand comprising the steps of completely carbonizing the resin at atemperature of about 1650 F. within a vacuum of about 20 microns,continuously exhausting the combustion gases produced by the burning ofthe resin, and thereafter cooling the heated molds to a temperature of400 F. prior to removal from the vacuum.

2. A method of increasing the transverse strength of a shell mold formedof a mixture, by weight, of 88% zircon sand and 12% phenol-formaldehyderesin which comprises the steps of completely converting the resin to anessentially pure amorphous carbon by subjecting the molds to atemperature of about 1650 F. within a vacuum of less than 20 microns,continuing the heating until the evolution of the combustion gases iscompleted, and thereafter cooling the heated molds to a temperature ofabout 400 F. prior to the removal thereof from the vacuum.

3. A casting mold characterized by relatively low permeability, hightransverse strength, and a virtual absence of contaminating effect onmolten refractory metals such as titanium and uranium, said moldconsisting essentially of a major proportion of zircon sand and a minorproportion of amorphous carbon surrounding the individual grains ofZircon sand uniformly throughout the mold, said carbon being the resultof the complete conversion, within a vacuum, of the polymerized resinutilized to temporarily bond the zircon sand into mold form.

4. A method of bonding a metal powder with substantially pure carbon toprovide a mold into which molten metal may be cast withoutcontamination, comprising the steps of coating the indifidual particlesof powder with a phenol-formaldehyde resin, polymerizing the resin totemporarily bind the powder into the desired shape of the mold, heatingthe mold to a temperature of about 1650 F. in a vacuum of 10 microns fora sufiicient period of time to completely convert the resin tosubstantially pure carbon, and cooling the heated mold to about 400 F.prior to removal from the vacuum to permanently bond the metal powderinto a material which will be relatively chemically inert when incontact with the molten metal being cast.

References Cited in the file of this patent UNITED STATES PATENTS2,548,897 Kroll Apr. 17, 1951 2,772,457 Webbers Dec. 4, 1956 3,023,119Anderson et a1. Feb. 27, 1962 FOREIGN PATENTS 187,247 Australia Oct. 25,1956 834,864 Great Britain May 11, 1960 OTHER REFERENCES The iron Age,November 23, 1961, pages 76 and 77.

1. A PROCESS FOR TREATING SHELL MOLDS FORMED OF A MIXTURE OF ZIRCON SANDAND A THERMOSETTING RESIN POLYMERIZED INTO A TEMPORARY BINDER FOR THESAND COMPRISING THE STEPS OF COMPLETELY CARBONIZING THE RESIN AT ATEMPERATURE OF ABOUT 1650*F. WITHIN A VACUUM OF ABOUT 20 MICRONS,CONTINUOUSLY EXHAUSTING THE COMBUSTION GASES PRODUCED BY THE BURNING OFTHE RESIN, AND THEREAFTER COOLING THE HEATED MOLDS TO A TEMPERATURE OF400*F. PRIOR TO REMOVAL FROM THE VACUUM.